1. Field of the Invention
The present invention relates to a deterioration detecting apparatus for a catalytic converter which purifies an exhaust gas from an internal combustion engine (hereinafter referred to as engine).
2. Description of the Prior Art
Conventionally, an exhaust gas from an engine has been purified by using a catalytic converter employing a three way catalyst to concurrently remove HC, CO, and NO.sub.x serving as injurious ingredients in the exhaust gas. Since a combustion efficiency of the engine is varied depending upon an air-fuel ratio of a mixture inhaled into the engine, feed back control of air-fuel ratio is carried out such that the air-fuel ratio becomes the optimal value (of, for example, 14.7) according to an operating state. In the feed back control, an air-fuel ratio sensor such as oxygen sensor has been conventionally mounted in an exhaust pipe of the engine at a position in the upstream of the catalytic converter, for example, at a portion of collected exhaust pipe manifolds, and the air-fuel ratio sensor has outputted a signal to carry out the feed back control of air-fuel ratio (hereinafter referred to as single air-fuel ratio sensor system).
However, since only one air-fuel ratio is provided in the single air-fuel ratio sensor system, variations in an output characteristic of the air-fuel ratio result in deterioration of control accuracy. Thus, it is necessary to compensate for the variations in the output characteristic of the air-fuel ratio sensor, and variations and time elapsing variations in parts such as fuel injection valve of the engine. For this purpose, there is proposed another control system (hereinafter referred to as double air-fuel ratio sensor system) in which a second air-fuel ratio sensor is mounted in the downstream of the catalytic converter to carry out double feed back of air-fuel ratio together with the upstream air-fuel ratio sensor (see, for example, U.S. Pat. No. 3,939,654).
For the following reasons, the double air-fuel sensor system has advantages in that a little variation is generated in the output characteristic of the air-fuel sensor in the downstream of the catalytic converter, and stable air-fuel ratio feed back control can be established.
(1) In the downstream of the catalytic converter, a low exhaust temperature can reduce a thermal effect.
(2) In the downstream of the catalytic converter, the catalyst can remove various injurious materials, resulting in less poisoning of the air-fuel sensor.
(3) In the downstream of the catalytic converter, the exhaust gas is sufficiently mixed, and an oxygen concentration in the exhaust gas has a value in a substantially equilibrium state.
A catalyst of the catalytic converter is designed so as to avoid considerable reduction of a function of the catalyst as far as a vehicle is used under a normal working condition. However, the function of the catalyst may be considerably reduced in case a user of the vehicle erroneously uses leaded gasoline as fuel, or in case a high tension cord is slipped off for some reason during operation of the vehicle to cause incapability of ignition. In the former case, the user never finds the error, and in the latter case, the user can drive the vehicle by simply correctly inserting the high tension cord. Therefore, the user frequently drives the vehicle without exchanging the catalyst so that the vehicle is running in a state where the deteriorated catalytic converter can not purify the exhaust gas.
In the double air-fuel ratio sensor system, when the function of the catalyst is deteriorated, an unburnt gas such as HC, CO, or H.sub.2 is exhausted downstream. Consequently, the downstream air-fuel ratio sensor is affected, and the output characteristic may be varied. As a result, there are problems of deterioration of fuel consumption, deterioration of driverbility, or deterioration of emission of HC, CO, and NO.sub.x.
Hence, there is proposed a deterioration detecting apparatus for catalytic converter to detect deterioration of the catalytic converter. For example, FIG. 1 is a diagram showing a conventional deterioration detecting apparatus for catalytic converter disclosed in Japanese Patent Publication (Kokai) No. 5-98949. In FIG. 1, reference numeral 1 designate an engine, 15 designates an exhaust pipe to discharge an exhaust gas from the engine 1, 12 is the catalytic converter containing the three way catalyst to concurrently purify the injurious ingredients such as HC, CO, NO.sub.x, in the exhaust gas, 10 is a first air-fuel ratio sensor mounted in the upstream of the catalytic converter 12 to generate an air-fuel ratio signal V.sub.1 according to the oxygen concentration in the exhaust gas, 11 is a second air-fuel ratio sensor mounted in the downstream of the catalytic converter 12 to generate an air-fuel ratio signal V.sub.2 according to the oxygen concentration in the exhaust gas, 111 is a catalyst deterioration detecting apparatus to detect deterioration of the catalyst in the catalytic converter 12, and 112 is alarm means for alerting when the catalyst is deteriorated.
A description will now be given of the operation. In the deterioration detecting apparatus for catalytic converter, the catalyst deterioration detecting apparatus 111 computes, for each of the air-fuel ratio sensors 10, 11, an area corresponding value S formed by the air-fuel ratio signals V.sub.1, V.sub.2 which are output voltage values from the air-fuel ratio sensors 10 and 11 mounted in the upstream and the downstream of the catalytic converter 12, and a predetermined voltage value. The catalyst deterioration detecting apparatus 111 computes, for each of the air-fuel ratio sensors 10 and 11, an inversion period T for which the output voltage value is inverted with respect to the predetermined voltage value. Further, the catalyst deterioration detecting apparatus 111 computes a catalyst deterioration deciding parameter by using the area corresponding value S and the inversion period T, or by combining both of them. The deterioration deciding parameter is compared with a predetermined value to decide whether or not the catalyst is deteriorated. When it is decided that the catalyst is deteriorated, the alarm means 112 alerts.
The conventional deterioration detecting apparatus for catalytic converter is provided as set forth above. Accordingly, there is a problem in that, when the catalyst is not sufficiently activated, decision of deterioration may be erroneously made even if the catalyst is not deteriorated.